Quantitative characterisation of H-Mordenite zeolite structure by infrared spectroscopy using benzene adsorption

Abstract

Infrared spectroscopy has been used to characterise the Brönsted acidity, the hydroxyls of H-Mordenite (HM) zeolite and the interaction of hydroxyls with benzene molecules. After pre-treatment under dry oxygen and then under vacuum at 723 K, three hydroxyl groups at 3749, 3660 and 3608 cm −1, assigned to terminal silanol, extraframework Al–OH and framework bridged Si–OH–Al species, respectively, have been detected. It shows that all the silanols, but only part of framework bridged protons and not the extraframework Al–OH groups can interact with benzene molecules. The number of framework bridged protons located in the main 12R channels, i.e. those accessible to benzene molecules, and that in the small channels are determined by a quantitative study of the changes of the adsorbance of the CH out-of-plane vibrations with benzene loading. It reveals that although the studied HM zeolite contains 4.6 framework bridged protons per unit cell, only around 1.5 of them are located in the main 12R channels. Others are in the small channels and are not accessible to benzene molecules. The acid strength of hydroxyl groups is evaluated by the shift of the hydroxyls (Δ ν OH) upon their interaction with benzene and compared with other protonic zeolites such as HZSM-5, HY, HBeta and HEMT. It indicates that the framework bridged OH groups of HM have a strong Brönsted acidity comparable with that of HZSM-5 zeolite.